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In some publications it is mentioned that a link local next-hop can’t be used when redistributing routes into BGP because routers receiving the route will not know what to do with the next-hop. That is one of the reason why HSRPv2 got support for global IPv6 addresses. One such scenario is described in this link.

The topology used for this post is the following.

I have just setup enough of the topology to prove that it works with the next-hop, so I won’t be running any pings and so on. The routers R1 and R2 have a static route for the network behind R3 and R4.

ipv6 route 2001:DB8:100::/48 GigabitEthernet0/1 FE80::5:73FF:FEA0:1

When routing towards a link local address, the exit interface must be specified. R1 then runs BGP towards R5, notice that I’m not using next-hop-self.

From my last post on PIM BiDir I got some great comments from my friend Peter Palúch. I still had some concepts that weren’t totally clear to me and I don’t like to leave unfinished business. There is also a lack of resources properly explaining the behavior of PIM BiDir. For that reason I would like to clarify some concepts and write some more about the potential gains of PIM BiDir is. First we must be clear on the terminology used in PIM BiDir.

Terminology

Rendezvous Point Address (RPA) – The RPA is an address that is used as the root of the distribution tree for a range of multicast groups. This address must be routable in the PIM domain but does not have to reside on a physical interface or device.

Rendezvous Point Link (RPL) – It is the physical link to which the RPA belongs. The RPL is the only link where DF election does not take place. The RFC also says “In BIDIR-PIM, all multicast traffic to groups mapping to a specific RPA is forwarded on the RPL of that RPA.” In some scenarios where the RPA is virtual, there may not be an RPL though.

Upstream – Traffic towards the root (RPA) of the tree. This is the direction used by packets traveling from source(s) to the RPL.

Downstream – Traffic going away from the root. The direction from which packets travel from the RPL to the receivers.

Designated Forwarder (DF) – A single DF exists for every RPA on a link, point to point or multipoint. The only exception as noted above is the RPL. The DF is the router with the best metric to the RPA. The DF is responsible for forwarding downstream traffic onto its link and forwarding upstream traffic from its link towards the RPL. The DF on a link is also responsible for processing Join messages from downstream routers on the link and ensuring packets are forwarded to local receivers as discovered by IGMP or MLD.

RPF Interface – The RPF interface is determined by looking up the RPA in the Multicast Routing Information Table (MRIB). The RPF information then determines which interface of the router that would be used to send packets towards the RPL of the group.

RPF Neighbor – The next node on the shortest path towards the RPA.

Sources in PIM BiDir

One of the confusing parts in PIM BiDir is how traffic travels from the source(s) to the RPA. There is no (S,G) and no PIM Register messages in PIM BiDir, so how is this handled?

When a source starts sending traffic it will send it towards the RPA regardless if there are any receivers or not. The DF on the segment is responsible for sending the traffic upstream towards the RPA. The packet then travels through the PIM domain until it reaches the root of the tree (RPA). In some articles on PIM BiDir, it is mentioned that there is no RPF check. This is not entirely true since RPF is used to find the right interface towards the RPA but it does not use RPF to ensure loop free forwarding, the DF mechanism is instead used for this.

The picture shows a source sending traffic to a multicast group, there are no interested receivers yet. Traffic from the source travels towards the RPA which is not a physical device but only exists virtually on a shared segment with the routers R1, R2 and R3. These routers are connected to the RPL and on the RPL, there is no DF election. This means that they are free to forward the packets, however, there are no receivers yet and hence no interfaces in the Outgoing Interface List (OIL). This means that traffic will simply get dropped in the bitbucket. This is a waste of bandwidth until at least one receiver joins the shared tree.

Considerations for PIM BiDir

Since there is no way to control what the multicast sources are sending, what we are giving up for getting minimal state in the PIM domain is bandwidth. It is not very likely though that a many-to-many multicast application will not have any receivers so this may be an acceptable sacrifice.

Due to having minimum state, PIM BiDir will use less memory compared to PIM ASM or PIM SSM, it will also use less CPU since there is less PIM messages to be generated and received and processed. Is this a consideration on modern platforms? It might be, it might not be. What is known though is that it is a less complex protocol than PIM ASM because it does not have a PIM Register process. Due to this, the RP, which does not even have to be a real router, can’t get overwhelmed by the unicast PIM Register messages. This also provides for an easier mechanism to provide RP redundancy compared to PIM ASM and SSM which requires anycast and MSDP to provide the same.

PIM BiDir Not Working in IOSv and CSR1000v

While writing this post I needed to run some tests, so I booted up VIRL and tested on IOSv but could not get PIM BiDir to work properly. I then tested with CSR1000v, both within VIRL and directly on an ESXi server with the same results. These images were quite new and it seems something is not working properly in them. When the routers were running in BiDir mode, they would not process the multicast and forward it on. Just a fair warning that if you try it that you may run into similar results, please share if you discover something interesting.

Putting it All Together

To get an understanding for the whole process, let us then describe all of the steps from when the source starts sending until the receiver starts receiving the traffic from the source.

The source starts sending traffic on the local segment to R4. R4 is the DF on the link since it’s the only router present.

R4 does a RPF check for 192.0.2.1 which is the RPA, through this process it finds the upstream interface and starts forwarding traffic towards the RPA where R1 is the RPF neighbor and the next router on the path towards the RPA.

R1 forwards traffic towards the RPA on its upstream interface but there are no interested receivers yet, so the traffic will get dropped in the bitbucket. Please note that both R2 and R3 does receive this traffic but if there is no interface in the OIL, the traffic simply gets dropped.

The PC then starts to generate an IGMP Report and sends it towards R5.

R5 is the DF on the segment which also means that it is the Designated Router (DR). It will generate a PIM Join (*,G) and send it towards the RPA on its upstream interface where R3 is the RPF neighbor. Only the DF and hence DR on a link may act on the IGMP Report.

R3 receives the PIM Join from R5 and since traffic is already being sent out by R1 on the RPL, R3 is allowed to start forwarding the traffic. Remember that there is no DF elected on the RPL. We now have end to end multicast flowing.

Conclusion

The main goal of this post was to show how the RP can be a virtual device on a shared segment. This means that redundancy can be designed into the RP role without any complex mechanisms used in PIM ASM. I also wanted to clarify some concepts with the forwarding and the terminology since there seem to be quite a few posts out there that are slightly wrong or not using the correct terminology.

This post will describe PIM Bidir, why it is needed and the design considerations for using PIM BiDir. This post is focused on technology overview and design and will not contain any actual configurations.

Multicast Applications

Multicast is a technology that is mainly used for one-to-many and many-to-many applications. The following are examples of applications that use or can benefit from using multicast.

One-to-many

One-to-many applications have a single sender and multiple receivers. These are examples of applications in the one-to-many model.

Scheduled audio/video: IP-TV, radio, lectures

Push media: News headlines, weather updates, sports scores

File distributing and caching: Web site content or any file-based updates sent to distributed end-user or replicating/caching sites

Announcements: Network time, multicast session schedules

Monitoring: Stock prices, security system or other real-time monitoring applications

Many-to-many

Many-to-many applications have many senders and many receivers. One-to-many applications are unidirectional and many-to-many applications are bidirectional.

Multimedia conferencing: Audio/video and whiteboard is the classic conference application

Synchronized resources: Shared distributed databases of any type

Distance learning: One-to-many lecture but with “upstream” capability where receivers can question the lecturer

Multi-player games: Many multi-player games are distributed simulations and also have chat group capabilities.

Overview of PIM

PIM has different implimentations to be able to handle the above applications. There are mainly three implementatios of PIM, PIM Any Source Multicast (ASM), PIM Source Specific Multicast (SSM) and PIM BiDirectional (BiDir).

PIM ASM

PIM ASM was the first implementation and is well suited for one-to-many applications. ASM means that traffic from any source to a group will be delivered to the receiver(s). PIM ASM uses the concept of a Rendezvous Point Tree (RPT) and Shortest Path Tree (SPT). The RPT is a tree built from the receiver to a Rendezvous Point (RP). The tree from a multicast source to a receiver is called the SPT. Before the receiver can learn the source and build the SPT, the RP will have sent a PIM Join towards the source to build the SPT between the source and the RP. When looking in the mroute table, RPT state will be shown as (*,G) and SPT state will be shown as (S,G)

Join the SPT and the RPT so the receivers get traffic and find out the source of the multicast

Initially traffic flows through the RP, there is a more efficient path though. When the Last Hop Router (LHR) starts receiving the multicast it will switch over to the SPT.The SPT will be a more optimal path and (likely) introduce lower delay between the source and the receiver.

PIM ASM can support both one-to-many and many-to-many applications since it can use both SPT and RPT. To prevent LHR to switch to SPT, ip pim spt-threshold command can be used. It can either be set to switch over at a certain rate of traffic (kbps) or be set to infinity to always stay on the RPT. This can be combined with ACL to have certain groups always stay on the RPT and for others to switch over. PIM ASM can therefore use SPT for some groups and RPT for other groups. There are still drawbacks to PIM ASM, a few are mentioned here:

Complex protocol state with Register messages

Redundancy requires the use of MSDP

Any source can send which opens attack vector for DoS and sending traffic from spoofed source

PIM SSM

PIM SSM was created to work better with one-to-many flows compared to PIM ASM. In PIM SSM, there is no complex handling of state and there is only SPT, no RPT. That also means that there is no need for a RP. PIM SSM is much easier to setup and use, it does require clients to support IGMPv3 so that the IGMP Report can contain which source the receiver wants to receive the traffic from. This also means that since there is no RP, there has to be some way for the receiver to know which sources send to which groups. This has to be hanled by some form of Out Of Band(OOB) mechanism. The most common use for SSM is IP-TV where the Set Top Box (STB) receives a list of sources and groups by contacting a server.

The drawback of PIM SSM is that (S,G) state is created requiring more memory. Depending on the number of sources, this may be a factor or not.

PIM BiDir

Bidirectional PIM was created to work better with many-to-many applications. PIM BiDir uses only RPT and no SPT. This means that there has to be a RP. With bidirectional PIM, the RP does not perform any of the functions of PIM ASM though, such as sending Register Stop messages or joining the SPT. Remember, in PIM BiDir, there is no SPT.The RP in PIM BiDir does not have to be a physical device since the RP is not performing any control plane functions. It is simply a way of forwarding traffic the right way, think of it as a vector. The RP can be a physical device and in that case it is a normal RP, just without the responsibilities of an RP as we know it in PIM ASM. When configuring PIM BiDir to have redundant RPs the RP is sometimes called Phantom RP, because it does not have to reside on a physical device.

PIM BiDir is often used in “hoot n holler” and financial applications. PIM BiDir and PIM SSM are at different ends of the spectrum where PIM ASM can serve both type of applications.

PIM uses the concept of Reverse Path Forwarding (RPF) to ensure loop free forwarding. RPF ensures that traffic comes in on the interface that would be used to send traffic out towards the source. PIM BiDir can send traffic both up and down the RPT. This is not normally supported by using RPF, to support this PIM BiDir uses a Designated Forwarder (DF) on each segment, even point-to-point segments. The main responsibility of the DF is to forward traffic upstream towards the RP. The DF is elected based on the metric towards the RP, essentially building a tree along the best path without having to install any (S,G) state. RPF is still used to find the appropiate path towards the Rendezvous Point Link (RPL) but it is the DF mechanism that ensures loop free forwarding.

RP Considerations

In PIM BiDir there isno MSDP, it does not use (S,G) so this is expected. To provide redundant RP in PIM BiDir, Phantom RP is used. The Phantom RP is a virtual RP which is not assigned to a physical device, it is often implemented by having two routers use a loopback with different subnet mask length.

Routers are assigned the RP adress of 192.0.2.1 which is then the Phantom RP, the actual routers where the traffic will flow through have been assigned 192.0.2.2 and 192.0.2.3 but with different net mask lengths. Normal best path rules will then forward traffic towards the longest path match which will be RP1 when it is available and RP2 when RP1 is not available. It is important to not configure the RP address as a physical interface address since this would break the redundancy. If a router was configured with the real address, it would not forward the traffic since the traffic would be destined for one of its own addresses.

Since the RP is so critical, redundancy must be provided. All traffic will pass through the RP which means that certain links in the network may have to carry a lot of the traffic. For this reason it can be necessary to have several RPs, that are acting as RPs for different multicast groups. The placement of the RP also becomes very important since traffic must flow through the RP.

PIM BiDir Considerations

PIM BiDir uses the DF mechanism and for the election to succeed, all the PIM routers on the segment must support PIM BiDir, otherwise the DF election will fail and PIM BiDir will not be supported on the segment. It is possible to have non PIM BiDir routers on a segment if a PIM neighbor filter is implemented to not form PIM adjacencies with certain routers. That way PIM BiDir can be gradually implemented into the network.

Closing Thoughts

PIM ASM supports all multicast models but at the cost of complexity. One could say that it’s a jack of all trades but does not excel at anything. PIM SSM is less complex and the best choice for one-to-many applications if the receivers have support for IGMPv3. PIM BiDir is best suited for many-to-many applications and keeps the least state of all the PIM implementations. Every PIM implementation has its use case and as an architect/designer its your job to know all the models and pick the best one based on business requirements.

Cisco VIRL is a great tool but it is artificially limited to a maximum of 15 nodes today. I have created a petition to collect names to send to Cisco, to show that the community really wants to increase this limit to at least 30 nodes.

Please go sign the petition if you are interested in seeing VIRL get support for more than 15 nodes.

I am currently studying for the CCDE exam. Elaine Lopes is the program manager for the CCDE and CCAr certification. I’ve had the pleasure of interacting with her online and meeting her at Cisco Live as well. The CCDE is a great certification and I wanted you to get some insight into the program and ask about the future of the CCDE. A big thanks to Elaine and Cisco for agreeing to do the interview.

Daniel: Hi Elaine, and welcome. It was nice seeing you at Cisco Live! Can you please give a brief introduction of yourself to the readers?

Elaine: Hi, it was nice to see you, too! My name is Elaine Lopes and I’m the CCDE and CCAr Certification Program Manager. I’ve been with Cisco’s Learning@Cisco team since 1999, – I’m passionate about how people’s lives can change for the better through education and certification.

Daniel: Elaine, why did Cisco create an expert level design program? What kind of people should be looking at the CCDE?

Elaine: Cisco has very well established expert-level certifications for network engineers in various fields which assess configuration, implementation, troubleshooting and operations skills; however, these certifications were never aimed to assess design skills. The root cause of many network failures is poor network design and the CCDE helps to fill this gap. The certification was created to assess a candidate’s skills in real-life network design. The candidate should mainly be able to meet business requirements through their network designs as well as understand design principles such as network resiliency, scalability and manageability! CCDE focuses on design by making technology decisions and justifying the choices made. Since CCDE is meant to assess design skills, it targets infrastructure network designers.

Daniel: Are there any prerequisites before taking the CCDE?

Elaine: There are no pre-requisites for the CCDE certification, although it is recommended that candidates have 7+ years of experience on network design in diverse environments.

Daniel: What kind of experience should a candidate have to be a good fit for the CCDE? What is the technology range that needs to be covered such as RS, SP, datacenter, security?

Elaine: CCDE is a role-based certification, and therefore it is desirable that candidates have experience (breadth and depth) in large-scale network designs, as they will be tested on making design decisions within constraints. CCDE focuses mainly on Layer 3 control plane, Layer 2 control plane and network virtualization technologies, but also assesses QoS, security, network management with a little of wireless, optical and storage technologies.

Daniel: Can you give us a short description of the exam process and at which locations the exam is available?

Elaine: The CCDE certification is divided into two steps. The first step is the CCDE written exam, which focuses on design aspects of the various technologies described above, and can be taken at any Pearson VUE testing center at any time. Once CCDE candidates pass the written exam, they then need to pass the CCDE practical exam, which is made of four different scenarios where technologies and design concepts are interconnected. The CCDE practical exam is tridimensional: the same technologies tested on the CCDE written exam plus the different job tasks (merge/divest, add technology, replace technology, scaling and design failure), and the task domains (analyze requirements, design, plan for the design deployment, and validate and optimize network designs). The CCDE practical exam is administered four days a year at any of the 275 Pearson Professional Centers (PPCs) worldwide.

Daniel: You are also responsible for the CCAr program. What is the difference between design and architecture? What kind of candidates should be looking at this exam?

Elaine: CCArs collaborate with senior leadership to create a vision for the network, and their outputs are the business and technical requirements which will be input for CCDEs to create a network design that meets these requirements. The pre-requisite for CCAr is to be a CCDE in good standing, with the target audience being the infrastructure architects who navigate between the technical and business worlds.

Daniel: What kind of study resources are available for the CCDE? I know you have been working hard on providing guidance in the written blueprint, what else is coming?

Elaine: The biggest challenge for CCDE candidates seems to be how to get started, so we recently launched the Streamlined Preparation Resources. This site offers a study methodology and links to many recordings with information about the CCDE program. It’s mainly a list of preparation resources that can be personalized for one’s own needs and offers diverse resource types in a very prescriptive way for CCDE candidates to prepare for the CCDE written exam, but also can be helpful for the practical exam. Since the CCDE practical exam is situation-based, the team decided to provide materials to make candidates think as network designers. The materials are not mapped 1:1 to the blueprint but our long-term objective will be to release the materials in bits and pieces as they come available.

Daniel: Elaine, tell us a bit about the upcoming CCDE study guide. Why was this book written and how should it be used to prepare for the CCDE?

Elaine: Marwan [Al-Shawi] approached me saying he wanted to author a CCDE book, so we had some conversations and exchanged many emails which helped shape the book outline, aiming to be an “all-in-one” study guide for the CCDE practical exam. He then went through the whole publishing process with Cisco Press. There are great technical reviewers involved in it, and the book is to be released soon – I can’t wait to get my copy!

Daniel: After my last blog post on the CCIE program, I received some comments where people questioned the integrity of the CCIE exam. How do you work with the integrity of the CCDE?

Elaine: Integrity has always been top of mind when considering the delivery of the CCDE practical exam: it’s Windows-based and administered at the secure PPCs. The exam changes between administrations, and the nature of the scenarios makes it hard to guess the responses.

Daniel: I know you have designed the CCDE to be as timeless and generic as possible while still covering the relevant technologies. How will the exam be affected of new technologies and forwarding paradigms, such as SDN?

Elaine: True. I’d expect the CCIEs and CCDEs out there to be at the forefront of adoption of these new technologies in the field, and we’re already making plans for incorporating these new technologies into the CCIE tracks. CCDE will be no different, but I don’t have details yet I can share.

Daniel: Creating exams is very difficult and people often have opinions on the material being tested. It’s not well known that you can comment on the exam while taking it. Isn’t it true that comments are one of your sources of feedback on the quality of the exam?

Elaine: I heavily rely on statistical analysis to understand both item and exam performance before making any adjustments to the exam.. To get a more holistic view, I also read the comments candidates make on items while taking the exam. These comments sometimes provide good insight on how to fix low-performing items.

Daniel: Elaine, how can people give feedback on the program outside of comments while taking the exam?

Elaine: Just send me an email elopes@cisco.com. To be informed on what’s going on in the CCDE world, you can connect with me on LinkedIn (Elaine Lopes) and/or Twitter (@elopes01).

Daniel: There is a Subject Matter Expert (SME) recruitment program for certifications within Cisco. Do you have SMEs for the CCDE and how can any CCDE’s out there contact Cisco if they want to be part of the SME program?

Elaine: SMEs are critical to assure the exams are relevant, so yes, I do have several CCDE certified SMEs participating on the various phases of the exam, from development teams to authoring/reviewing/editing items, to working on the preparation resources, to the maintenance of the existing exams, etc. If you are CCDE-certified and want to participate, join the program and I’ll contact you for the next opportunity to participate.

Daniel: Thank you so much for your time, Elaine! I hope we’ll meet soon again. Do you have any final words and where do you see the CCDE program going in the future?

Elaine: I wanted to give a hint to CCDE candidates taking the practical exam: take the time to read and connect with each scenario and don’t make decisions or assumptions outside the context of the scenario. If you read a question and the answer is not glaring, go back to the scenario materials! CCDE design principles don’t change, so when the time comes I see “sprinkling” design aspects of new technologies in the CCDE exams. Hope to see you soon!! It’s been a pleasure to participate, thank you for inviting me!